The discovery of the fluidized bed process for the production of linear olefin polymers provided a means for producing these diverse and widely used polymers with a drastic reduction in capital investment and a dramatic reduction in energy requirements as compared to then conventional processes.
To be commercially useful in a gas phase process, such as the fluid bed processes of U.S. Pat. Nos. 3,709,853; 4,003,712 and 4,011,382, all of which are incorporated herein by reference; Canadian Pat. No. 991,798 and Belgian Pat. No. 839,380, the catalyst employed must be a highly active catalyst. Typically, levels of productivity reach from 50,000 to 1,000,000 pounds of polymer or more per pound of primary metal in the catalyst. High productivity in the gas phase processes is desired to avoid the expense of catalyst residue removal procedures. Thus, the catalyst residue in the polymer must be small enough that it can be left in the polymer without causing any undue problems to either the resin manufacturer, or to a party fabricating articles from the resin, or to an ultimate user of such fabricated articles. Where a high activity catalyst is successfully used in such fluid bed processes, the transition metal content of the resin is on the order of .ltoreq.20 parts per million (ppm) of primary metal at a productivity level of .gtoreq.50,000 pounds of polymer per pound of metal. Low catalyst residue contents are also important in heterogeneous catalysts comprising chlorine containing materials such as the titanium, magnesium and/or aluminum chloride complexes used in some so-called Ziegler or Ziegler-Natta type catalysts. Use of these heterogeneous catalysts results in a polymerization reaction product which is a complex mixture of polymers, with a relatively wide distribution of molecular weights. This wide distribution of molecular weights has an effect (generally detrimental) on the physical properties of the polymeric materials, e.g. decreased tensile strength, dart impact.
The molecular weight distribution (MWD), or polydispersity, is a known variable in polymers which is described as the ratio of weight average molecular weight (Mw) to number average molecular weight (Mn) (i.e., Mw/Mn), parameters which can be determined directly, for example by gel permeation chromatography techniques. The I.sub.10 /I.sub.2 ratio, as described in ASTM D-1238, can be an indicator of the MWD in conventional heterogeneous ethylene polymers. The I.sub.10 /I.sub.2 ratio is also an indicator of the shear sensitivity and processibility for ethylene polymers. Low density polyethylenes (LDPE) typically have a higher I.sub.10 /I.sub.2 ratio than linear low density polyethylenes (LLDPE) or ultra low density linear polyethylenes (ULDPE) and are easier to melt process in fabrication equipment at comparable I.sub.2 values.
Ethylene polymers having a narrow MWD and homogeneous comonomer distribution are known. These polymers can be produced using "single site" catalysts, such as metallocene or vanadium catalysts. While the physical properties of these polymers are generally superior to heterogeneous polymers, they are often difficult to process with conventional melt fabrication equipment. The problems are manifested, for example, in their lack of ability to sustain a bubble in a blown film process, and by a "sag" when evaluated in blow molding processes. In addition, the melt fracture surface properties of these polymers are often unacceptable at high extrusion rates, a feature that makes them less desirable for use in equipment operating at current high speed extrusion (i.e., production) rates. Extruders often exhibit increased power consumption due to the low shear sensitivity of these polymers.
Use of the catalyst systems described in U.S. Pat. No. 5,374, 696 and 5,453,410, both of which are incorporated herein by reference, results in the production of unique polymers having the properties as taught in U.S. Pat. No. 5,272,236 and U.S. Pat. No. 5,278,272, which are incorporated by reference. These polymers are substantially linear olefin polymers which are characterized as having a critical shear rate at the onset of surface melt fracture of at least 50 percent greater than the critical shear rate at the onset of surface melt fracture of a linear olefin polymer having about the same I.sub.2 and Mw/Mn.
There is a need for a gas phase olefin polymerization catalyst that can be used more efficiently and effectively to polymerize or copolymerize ethylene with higher alpha-olefins, e.g. alpha-olefins having 3 to 20 carbon atoms. In practice, the commercial copolymers are made using monomers having only 3 to 8 carbon atoms (i.e., propylene, 1-butene, 1-hexene, 1-octene and 4-methyl-1-pentene) because of the low rate of reactivity and incorporation of the alpha olefins with larger carbon chains and, for gas phase processes, because of the lower concentration possible in the reactor for alpha-olefins with larger carbon chains. The traditional Ziegler catalysts are not particularly efficient or effective in incorporating the higher alpha-olefin comonomers into the polymer. The rate of reaction for the ethylene monomer is much greater than the rate of reaction for the higher alpha-olefin monomers in the copolymerization reaction using traditional multi-site Ziegler catalysts. Accordingly, due to the lower reaction rate of incorporating the longer chain comonomer into the growing polymer chain, the copolymer fractions containing the higher alpha-olefin comonomers are generally the lower molecular weight fraction having limited desirable physical properties. These factors also contribute to polymer particles sticking together or agglomerating in the gas phase process.
Even in the most current olefin copolymerization systems, there is still a need for a gas phase olefin polymerization catalyst which is able to incorporate efficiently larger amounts of alpha-olefins into a copolymer chain and give a polymeric product which has a narrow molecular weight distribution and is more homogeneous with respect to comonomer distribution than otherwise would be achieved using a Ziegler catalyst under comparable conditions. The properties and advantages of linear homogeneous copolymers are described in U.S. Pat. No. 3,645,992 which is incorporated herein by reference.
Canich et al. teach in U.S. Pat. No. 5,057,475, U.S. Pat. No. 5,026,798, and U.S. Pat. No. 5,096,867, all of which are incorporated herein by reference a supported catalyst system which includes an inert support material, a Group IV B metal component and an alumoxane component for use in the production of high molecular weight polyolefins. While Canich et al. teaches the use of their catalysts under various reaction conditions, the gas phase examples of their '475 patent report polymer products having a relatively broad MWD (e.g. &gt;2.9).
There is also a need for a gas phase process to produce more homogeneous narrow molecular weight distribution polyolefins (MWD of 1.5-2.9), that have improved processability such as provided by substantially linear olefin polymers.